Phakopsora pachyrhizi Syd. & P. Syd
Phakopsora pachyrhizi Syd. & P. Syd is the most virulent species that causes rust on soybeans (Glycine max). This basidiomycete was first found in 1902 in Japan and was named Uredo sojae Henn. It was only in 1914, that it was described in Annual Mycology and renamed Phakopsora pachyrhizi Syd. & P. Syd by Hans and Paul Sydow (8, 13). Soybean is one the most important commodities grown worldwide and is highly affected by Phakopsora pachyrhizi. It is economically important because it is a cheap source of protein and good source of oil. The main use of soybean is for animal feed and secondary for human consumption (7). For many decades P. pachyrhizi was spread through Asia and Australia and only in 1994 it was observed in Hawaii. After that it was found in the African continent and in 2001 reported in Paraguay. Since then P. pachyrhizi has spread to many other countries in South (Brazil, Uruguay, Argentina, Colombia, Bolivia) and Central America (Puerto Rico), where it has caused a significant amount of crop loss (8, 13). In 2004 the fungus was observed in the USA, close to Baton Rouge (LA) causing little damage. The primary explanation for the dissemination of the disease from South America to USA is the occurrence of Hurricane Ivan (17). This fungus has many synonyms like: Phakopsora sojae Fujikuro, Malupia sojae (uredial anamorph), Phakopsora calothea H. Sydow and Uredo sojae P. Hennings. In taxonomy it belongs to the Basidiomycota phylum, Urediniomycetes class, Uredinales order, and Phakopsoraceae family (8). Until 1992 soybean rust was only attributed to the fungus Phakopsora pachyrhizi. It was only after that, that researchers discovered that Phakopsora meibomiae can also cause the disease. Although, the latter species is less aggressive (2). In middle or late summer, symptoms of soybean rust show up 4 to 5 days after inoculation. This fungi produces small, tan to brown colored and vein delimited lesions on the bottom side of the leaves. Small bumps called pustules develop on top of the lesions (5). These pustules are called sori, but can be given many other different names (18). Plant lesions reduce the photosynthetic area of leaves, and consequently reduce plant yield potential. In the early stages of soybean rust, the lesions can look very alike to symptoms of some bacterial diseases. P. pachyrhizi and P. meibomiae are very alike but there are some features that can be used to distinguish them. P. pachyrhyzi’s teliospores are irregularly distributed in layers of 2 to 7 spores. Spore walls are yellow or hyaline and 1.0 mm thick, but can get up to 3.0 mm on the most external spore layers. P. meibomiae’s teliospores are also irregularly distributed in layers but of 1 to 5 spores. Their walls are cinnamon to light colored brown and 1.5 to 2 μm thick, but can get up to 6 μm on the external layers (16). The size of uredospores can range from 18- 34 to 15-24 microns and are pale yellowish-brown colored (14). They are also sessile and have 1 micron thick walls. Solar radiation can reduce uredospores viability, but germinating in clumps can protect the spores from desiccation (9). Spermogonia and aecia were not observed in P. pachyrhizi (13). P. pachyrhizi, just like the other rust fungi, is an obligate biotroph pathogen and cannot be cultured in media. It only survives in living plant tissues in a parasitic phase and not in plant debris or dried tissue (10). Rust fungi can have up to 5 different stages in its lifecyle. But because of lack of information about P. pachyrhizi’s sexual stage, the production of all five has not been confirmed (8). In stage zero, spermatia are not observed and in stage 1 aeciospores are not observed. During stage two, uredospores are common and uredinia are growing on the bottom side of the leaves and in stage three teliospores can be observed but are not very common to be present. In the final stage (four) basidiospores can be identified. Therefore, P. pachyrhizi is a microcyclic fungi because it only produces uredospores, teliospores and basidiospores. (2, 4). Hardy windborne uredospores (n + n) are the primary disease propagule and can germinate in clumps and get disseminated over very long distances and initiate new infections (11). These uredospores germinate and produce a telium (n + n), which will produce the teliospores (n + n) (2). Teliospores are the sexual and overwintering structure but it has never been observed to germinate (8). There is a lack of information about teliospores’ role in the disease process. When germinating, Phakopsora spp. uredospores show appressorium-mediated direct cuticle penetration (1). Continuous production of uredospores on alternative hosts, like kudzu, is how the pathogen survives between soybean production seasons. (15). The role of dissemination and infection of uredospores is very complex. There are several steps in uredospores attachment and enzymes play an important role in changing host cuticule composition. An example of enzymes, are the cutinases released by the pathogen and it changes the host cuticle so that the pathogen can attach its adhesion pad to the surface (6). Later on, appressorium is formed and the penetration hyphae develops and forms the substomical vesicle under the plant tissue’s guard cells. Hastorium are then formed to start the nutrient uptake (18). Phakopsora pachyrhizi is an autoecious fungus. It completes its lifecycle on one host, which can be a soybean plant or any other alternative host (2). Furthermore, soybean rust is a polycyclic disease, P. pachyrhizi is capable of having several infection cycles per season and can spread very quickly (11). The best habitat where it is found is usually humid tropical and subtropical regions and it is present in Asia, Australia, South and North America (Hawaii and southeastern U.S.) and Africa. Soybean rust can spread very quickly to different regions and start an epidemic disease mainly because of three factors: uniform susceptibility in the host, conducive environment and the introduction of the new pathogen. According to Marchetti et. al. (1975) (12), the optimum environmental conditions for uredospores to germinate are temperatures between 15 and 25°C, and 20 to 25°C for infection, with 6 to 12 hours of continuous leaf wetness and high relative humidity. Rust fungi are usually grouped into genera that are often monophyletic. The P. pachyrhizi’s family can be distinguished by their teliospores or by their probasidium being unstalked (8, 18). Most species of the Uredinales Order have a very complex lifecycle, which involves the production of many different spore types. This is one of the main reasons why it is difficult to classify species and make phylogenetic inference within the order (8). Unfortunately there is limitation in the use of modern molecular characters to rust systematics (3). Soybean rust has been a serious problem mostly in South America where the three factors of the disease triangle are present (conducive environment, presence of the pathogen and susceptible host). P. pachyrhizi can cause up to 80% in yield loss and in the 2001-2002 soybean crop season in Brazil, total grain losses due to soybean rust reached US$ 125.5 million. (20). To reduce soybean rust yield loss, regular applications of fungicides (chemical control) and the use of resistant varieties (cultural control) are essential. Applications of appropriate fungicides should start prior or in the beginning of the infection, to suppress pathogen spread. Breeding for resistant varieties has gain importance in the past years and it has been conducted by classical germoplasm screens (8). Currently, the objectives in research are to find soybean genes that are resistance to high virulent populations of P. pachyrhizi‘s (19) In the U.S. quarantine has also controlled the spread of P. pachyrhizi. This is important because the pathogen can infect many alternative hosts (31 species in 17 genera), like kudzu (Pueraria montana var. lobata), lime beans, cowpeas, clovers and other legume species (8), but alternate hosts are unknown. On these alternative hosts, P. pachyrhizi can build up inoculum when soybean plants are not around, making it very difficult to eradicate this fungus.
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